Perhaps the leading use for liquid level sensing technology is in the area of motor vehicle fuel level detection. Motor vehicle fuel level detectors or sensors are subjected to a dynamic and harsh environment within the confines of a motor vehicle fuel tank. While such fuel level sensors generally are not required to indicate fuel levels with great precision, it is required that they do so reliably despite the dynamics of the environment.
Presently, motor vehicle fuel level sensors tend to be located within a cylinder inside the vehicle fuel tank, in close proximity to the vehicle fuel pump. Fuel level sensors tend to be vertically oriented resistive elements about which a float with a conductive element resides. Float position, relative to the resistive element, changes according to the level of liquid in the cylinder.
U.S. Pat. No. 4,702,107 is fairly representative of the technology of modern vehicle fuel level sensors. This patent discloses a rectangular vertically oriented insulating material contained within a cylindrical container, housed within a vehicle fuel tank. The insulating material has two primary functional sides. On one side two variable resistive elements run parallel longitudinally and are connected by transverse conductive elements. On the other side conductive tracks run substantially parallel to the resistive elements on the first side. A float is disposed about the vertically oriented insulating material. The float contains a fixed metal element shaped to coincidentally contact the transverse conductive elements attached to the variable resistive elements on one side and the parallel conductive tracks on the other. Upper ends of the variable resistive elements and conductive tracks are adapted to connect to an external electric circuit. With a DC voltage applied, the float position corresponding to the level of liquid in the tank, establishes a current path down through the portion of resistive elements above the float, across the metal element in the float, up the conductive track above the float to the external electric circuit. Consequently, resistance in the current path is inversely proportional to the level of liquid in the tank.
Certain level indicator meters, which attach to level sensors, make it desirable to generate a resistance directly proportional to the level of liquid in a tank (i.e. a higher fuel level corresponding with a higher resistance). A variable resistor with float configuration can facilitate this direct proportionality if the external circuit applying the DC voltage is connected to the bottom of the resistive element on the vertically oriented insulator. Such a connection results in a current path down a conductor from the external circuit to the bottom of a resistive element which is submersed in fuel. Current then flows up the resistive element to a metal element in the float, across the float to a conductive track and up to the external circuit. The higher the float is up the resistive element, the greater the resistance that current flow is subject to.
Difficulties arise in extending a conductor to the bottom of a submerged vertical insulator for connecting to a resistive element thereon. A separate conductor running the length of the vertical insulator presents a potential obstacle to a float which must slide uninhibitedly up and down the insulator. The separate conductor, which represents an additional part to be installed in some manner, adds cost to the assembly. Similarly, the additional conductor presents a potential source of discontinuity if worn or broken, detracting from the level of reliability of a sensor.
A conductor disposed on a surface of an insulator, parallel with a resistive element and connected with the resistive element at the bottom of the insulator presents an equally vexing problem. In a tank containing fuel contaminated with water, which is not miscible with hydrocarbon fuels, the water settles to the bottom of the tank because it is heavier than hydrocarbon fuels. Submerging an insulator with surface disposed conductors in water concentrated at the bottom of a fuel tank and applying a DC voltage, results in an electrolytic reaction wherein ions from the higher potential conductor connected to the resistor element migrate to the lower potential conductive track. Eventually the conductor connected to the resistive element fully erodes opening the circuit and rendering the fuel level sensor inoperable.